Superspreading of aqueous films containing trisiloxane surfactant on mineral oil

The spreading of water droplets, containing various surfactants, on liquid mineral oil surfaces has been investigated. Only "superspreading" trisiloxane surfactants were observed to promote rapid spreading on mineral oil, and the spreading characteristics of these systems differ substantially from those observed on solid substrates of comparable hydrophobicity. These differences include significantly faster spreading rates and monotonically increasing spreading rates with increasing surfactant concentration. Superspreading is attributed to Marangoni flow along the mineral oil-water interface. The extremely fast spreading on the mineral oil surface, as compared to hydrophobic solid surfaces, is attributed to the absence of the no-slip condition at the mineral oil-water interface. Real-time videomicroscopy of the spreading of aqueous droplets containing the trisiloxane surfactant M(D'E₄OH)M revealed stepwise motion of the leading edges of the aqeuous drop driven by disintegration of large surfactant aggregates at the mineral oil-water interface. This observation suggests that disintegrating aggregates instantaneously deliver large amounts of surfactant to the mineral oil-water interface, creating large surface tension gradients that are required for Marangoni flow. These results imply an important role of aggregate disintegration during the spreading of surfactant dispersions on hydrophobic solid substrates.